MATERIAL HANDLING— TRANSPORTATION OF SOLIDS 85
the average cost of a 5-ton, medium-speed, crane of 25-ft. span taken as 1 are given in Table 6. This data provides sufficiently reliable values for comparing the cost of various sizes of cranes and for purposes of estimating the probable cost of the equipment, etc.
Base: 5-ton, medium-speed, crane of 25-ft. span = 1.
Depreciation of overhead electric cranes is customarily figured at 5 per cent of cost — a safe value — while ordinary repairs should not average more than 2 per cent. Interest on investment, insurance, taxes, etc., add another 8 per cent of the initial cost of the equipment per year, making the average burden 15 per cent. One man to run the crane with some occasional service from an assistant constitutes the necessary operating crew, the expense of which should not average per hour more than one and three-seventh times the hour-wage rate of the craneman. Oil, waste and incidental supplies, including the expense of an occasional careful overhauling and thorough cleaning of equipment, varies closely with the total power consumption of the crane and may be taken at a mean value of 0.15 ct. per total motor horsepower per hour. The average power requirements, including the power required to run the crane empty as well as under load is in kilowatts per hour taken as equal to half the total horsepower requirements of the crane expressed in horsepower per hour.
The total net cost of operating the crane for a period of a year should be the basis of estimating the net operating cost per hour or per ton handled, in order that the burden expense may be equitably apportioned. If this is not done, it is necessary to allot arbitrarily a certain proportion of the yearly burden to the operating expense for the period under consideration. The net cost of operating a standard overhead electric crane, expressed in the cost per ton handled and per hour of operation, follow as Formulas (6a) and (66) :
C/t = (P Formula (6a) C/hr * C/t X Q" Formula (6Z>)
P = Average power cost = consumption in kilowatts per hour times unit rate =
0.5 times total crane horsepower per hour times unit power rate, L = Labor charge per hour = 1.43 times hour-wage rate of craneman, E = Incidental expenses per hour = 0.15 times total crane horsepower per hour, T = Task duration (yearly) in hours,
B = Burden (yearly) = 0.15 times initial cost of equipment, Qf = Quantity of material handled in tons, and Q" = Quantity of material handled in tons per hour — bucket, or crane, with
An overhead electric crane is equipment which is rarely in continuous operation, so other conditions than simply its lifting capacity govern, as a rule, its selection. The maximum load to be handled must be known, total tonnage to be handled per year estimated and the number of probable operating hours established. The service required may be of a special nature, such as handling a heavy load once, or a few times per year, in which case the lifting capacity of the crane would be the one governing factor. Or, a quantity of product or other commodity measured in tons per year might have to be handled, complicating the selection of the economic size of crane to install. The selection in such case, though allowance for expansion should always be made, entails a knowledge of all governing factors and is best illustrated by considering a typical example — setting down in detail the steps customarily taken.
Example 3. — Required economic size and net operating cost of overhead electric crane to handle 120,000 tons of material per year (300 8-hr, working days), operating an average of 2 hr. per day. Specifications: 50 ft. span, 40 ft. lift (maximum), 240 ft. bridge travel (maximum); medium-speed class crane.